The letter is too powerful to throw away. It gets the better of you.[Drop Undyne Letter EX]

Main Story

If the protagonist spares Flowey after his battle in a Neutral Route, he tells the protagonist to backtrack and see Alphys. After reaching the bridge that connects the CORE and MTT Resort, Undyne calls the protagonist and tells them to come to Snowdin. In Snowdin, Undyne gives the letter to the protagonist.

If the protagonist defeats Flowey and has not befriended Undyne or Papyrus, Flowey instructs the protagonist to befriend the two characters. In this case, Undyne gives the protagonist the letter after they hang out. Befriending Undyne is only possible if the protagonist has gained no EXP.

When delivered to Alphys, she debates whether or not to read it before chastising herself, and opening it with some loud machine (possibly a chainsaw). At first, she believed the contents to be a joke being played on her and decides to confront the person who delivered it.[1]

Alphys becomes flustered when she realizes that the protagonist is on the other side of the door; she believes that the protagonist wrote the letter.[2] She then goes on a date with the protagonist.

Undyne's letter cannot be sold in the Tem Shop. Its price is replaced with "NO!G," and if the protagonist attempts to sell it regardless, one of Mettaton's soundbytes play.

References

↑"(O-oh n-no, is that another letter...?) (I don't want to open it...) (C-can't I just slide it back out...?) (N... no... I can't keep doing this.) (I'll read this one.)" - Alphys